Conventionally, filters such as an air filter, filtration filter and the like have been used widely in semiconductor manufacturing industry and other industries. In recent years, filters composed of polytetrafluoroethylene (PTFE) porous membrane with extremely low pressure loss and extremely high trapping efficiency have attracted attention.
The filters composed of PTFE porous membrane can be produced, for example, by the following method. First, a sheet-like molded body of un-sintered or semi-sintered PTFE is formed and then the resultant sheet-like molded body is stretched to form cracks, thereby forming pores in the sheet-like molded body to produce the filter.
However, since the pores are formed in the sheet-like molded body by forming cracks in the sheet-like molded body, it is difficult to control the pore diameter. Note that Japanese Patent Laid-Open Publication No. 2003-41083 discloses a technique to have PTFE contain carbon fiber.